The claimed invention relates generally to the field of motor driver circuits and more particularly, but not by way of limitation, to an apparatus and method for operating a disc drive spindle motor which supports a rotatable magnetic recording disc.
A disc drive is a data storage device used to store digital data. A typical disc drive includes a number of magnetic recording discs that are axially aligned and mounted to a spindle motor for rotation at a high constant velocity. The spindle motor is mounted to a base deck, which combines with a top cover to provide a sealed housing. A corresponding array of read/write heads access fixed sized data blocks on tracks of the discs to write data to and read data from the discs. In disc drives of the current generation, the discs are rotated at speeds of 15,000 revolutions per minute and more.
Disc drive spindle motors typically have a multi-phase, direct current (dc) brushless motor configuration. The phase windings are arranged about a stationary stator on a number of radially distributed poles. A rotatable spindle motor hub is provided with a number of circumferentially extending permanent magnets in close proximity to the poles. Application of current to the windings induces electromagnetic fields that interact with the magnetic fields of the magnets to apply torque to the spindle motor hub and induce rotation of the discs.
It will be recognized by those of skill in the art that slight variations exist in the operating velocity of a spindle motor during operation, and the importance of controlling and minimizing these variations is increased due to the high operating velocities currently employed in disc drive spindle motors. This importance is paramount due to the difficulty of aligning the read/write heads with data blocks as data is stored or retrieved from a disc.
Motor control circuitry typically uses a digital-to-analog converter (DAC) and field effect transistor (FET) drivers to control the spindle motor. The DAC receives digital reference signals and outputs a corresponding analog current or voltage over a selected dynamic range that modulates the amount of current applied by drive circuitry to the spindle motor. A typical unit of measurement for a DAC is volts per count.
Motor control circuitry may require different full-scale ranges of operation to accommodate different power requirements of the motor. For example, when the motor is at rest the motor needs a large input power to begin rotation. Conversely, much less input power is required to regulate the motor velocity when the motor is running near operating velocity.
The range of power requirements for the spindle motor also depends in part on the type of bearings used to support the spindle motor hub in rotation with respect to the base deck. For spindle motors that use ball bearings, variations in temperature do not greatly affect the friction force encountered by the spindle motor hub. However, fluid dynamic bearings use a lubricant to support the spindle motor hub for rotation with respect to the base deck. The viscosity of this lubricant may be highly temperature dependent such that power requirements to drive the motor vary greatly with temperature.
There is a need, therefore, for an improved disc drive motor control circuit that adaptively adjusts to various full-scale operating ranges to account for different spindle motor configurations and operating conditions.
In accordance with preferred embodiments, a disc drive data storage device includes a spindle motor which supports at least one magnetic recording disc. Motor control circuitry is configured to rotate the spindle motor at a desired operational velocity. The motor control circuitry includes a digital to analog converter (DAC) assembly which converts input digital signals to corresponding analog signals over a range of different selectable dynamic ranges.
The motor control circuit is preferably operated by initially identifying a first dynamic range of motor adjustment signals which can be initially applied to control operation of the motor. A first motor adjustment signal within the first dynamic range is applied to the DAC assembly to control application of current to the motor. The first motor adjustment signal is determined in relation to a detected motor velocity error.
When the first motor adjustment signal is determined to be proximate a selected one of an upper end or a lower end of the first dynamic range, the first dynamic range of the DAC is adjusted to provide a different, second dynamic range of motor adjustment signals. Thereafter, a second motor adjustment signal within the second dynamic range is applied to control application of current to the motor.
Preferably, the first motor adjustment signal is determined to be proximate the upper end of the first dynamic range when a magnitude of the first motor adjustment signal is between a maximum level of the first dynamic range and a first threshold level between the maximum level and a minimum level of the first dynamic range. The first motor adjustment signal is determined to be proximate the lower end of the first dynamic range when a magnitude of the first motor adjustment signal is between the minimum level and a second threshold level between the minimum level and the maximum level.
Expanding the dynamic range of the DAC allows greater amounts of current to be applied to the motor, which is particularly desirable when a spindle motor having hydrodynamic bearings is operated at a lower temperature. Contracting the dynamic range provides higher resolution (volts/count) and greater stability of the circuit, which is particularly desirable when the motor has achieved steady state operation. The different dynamic ranges are automatically selected for different motor load conditions and accommodate different mechanical configurations of the spindle motor.
These and various other features and advantages which characterize the claimed invention will become apparent upon reading the following detailed description and upon reviewing the associated drawings.